The present invention relates to an excellent far-infrared radiating material, and more particularly to a porous oxide surface layer derived from an Al--Mn alloy layer which has excellent far-infrared radiating characteristics. The porous oxide surface layer will hereinafter be referred to as an "Al--Mn alloy-derived oxide layer" or an "Al--Mn alloy-derived oxide surface layer", or merely as an "Al--Mn oxide surface layer".
Materials having the Al--Mn system oxide surface layer of the present invention can be used for performing heating and drying in a variety of industrial fields to achieve a higher efficiency, resulting in much savings of energy. Furthermore, due to its ease of working and light weight, a material having the surface layer of the present invention may be used in many new fields in industry.
Far-infrared radiation is a form of electromagnetic radiation having a wavelength of 3-1000 micrometers. Far-infrared radiation is adsorbed into water or into organic matter to generate heat, which is effective to achieve efficient heating and drying. It is also said that in addition to the above-described thermal effect, far-infrared radiation facilitates movement of water molecules.
The latter effect is similar to the effect that microwaves, which are the next to far-infrared rays in wavelength, exert on water molecules when used in a microwave oven. Aluminum foil cannot be heated in a microwave oven, since aluminum foil does not contain water. On the other hand, when water-containing materials such as foods are irradiated by microwaves in the oven, water molecules are vibrated to generate heat energy producing a local rise in temperature, and the heat is further transferred to the whole of the object to heat it as a whole.
Far-infrared radiation does not vibrate water molecules so vigorously as do microwaves, but far-infrared rays facilitate movement of water molecules so that evaporation of water is accelerated without raising its temperature. Furthermore, much water evaporates to remove heat of evaporation, which then effectively suppresses an excess rise in temperature in the surface of the heated material.
In order to practically apply such effects as thermal energy generation and water molecular movement which are efficiently promoted by far-infrared radiation, it is necessary to develop materials having excellent capability to radiate far-infrared rays. An example of such a material is a porous Al--Mn system oxide layer, which is manufactured in such a manner that an Al--Mn alloy plate (Mn: about 2%, Mg: about 0.5%, Fe: about 0.1%, Al: bal.) prepared from a molten metal through solidification, rolling, and annealing is subjected to an anodic oxidizing treatment by conventional electrolysis at a given electric current density in a sulfate solution. An oxide film is formed to a thickness of about 30 micrometers on the surface of the plate.
Although the thus-formed conventional Al--Mn oxide layer has been tried for heating and drying in various applications, it has the problem that radiation of far-infrared rays of the porous Al--Mn oxide film is degraded when the service temperature goes up beyond 350.degree. C.
FIG. 2 shows a relationship between the service temperature and radiation of far-infrared rays at a wavelength of 15 micrometers for the above-described conventional Al--Mn oxide layer. As is apparent from the graph, when the service temperature is over 350.degree. C., the radiation of far-infrared rays is markedly reduced.
It is not unusual for heating or drying to be carried out at a temperature above 350.degree. C., and therefore there are few applications in which conventional materials having an Al--Mn oxide layer can be used in conventional heating or drying apparatuses. Thus, there is a need for a material or element having an ability to generate far-infrared radiation which is not degraded substantially even when the service temperature is over 350.degree. C.